On Language Design: Semicolons or no semicolons?

_(This is a cross-post from the [mobl blog](http://www.mobl-lang.org/417/semi-colon/))_

When should I use semicolons (`;`) and when shouldn’t I in [mobl](http://www.mobl-lang.org)?

It’s a common question. Here’s the reasoning behind requiring semicolons in one context and none in the other.

In mobl, semicolons signify _imperative execution_, i.e. execution from top to bottom. For instance:

var n = 8;
n = n + 10;
n = n * n;
alert(n);

First create a variable `n`, and assign `8` to it, _then_ add 10 to `n`, then multiply `n` with itself and save the result to `n`, and _then_ show a pop-up dialog with the result.

A lot of mobl is not imperative but _declarative_, i.e. you don’t define a recipe of how to get from A to B, but instead you define the desired end-result and let mobl figure out how to realize it. An example of this is mobl’s user interface language:

screen root() {
header(“Welcome”)
group {
list(t in Task.all()) {
item { checkBox(t.done, label=t.name) }
}
}
}

Ostensibly you may think these are basically just a list of imperative statements. _First_ render a header, _then_ a group, _then_ loop over each item in `Task.all()`. However, you shouldn’t think of it this way. While this may be the execution order in an initial render, parts of the user interface may _react_ to application state changes later, e.g. when new tasks are added to the `Task.all()` collection. As a result, the user interface may change a lot after the initial render. In a user interface you define _what_ you want, not how to realize this vision, unlike _script_ where you spell out the steps to a desired result.

To make this conceptual difference clear in the syntax, mobl uses semicolons. _Imperative_ code in mobl uses semicolons, _declarative_ code does not.